A cool, red star of spectral type M with a surface temperatures of
less than 3600 K. The spectra of M stars are dominated by molecular
bands, especially those of TiO. Naked-eye examples are Betelgeuse and
Antares.

A → stellar magnetic field associated with
a → massive star.
Magnetic fields are detected only for seven to ten percent of all
studied massive → OB stars, and the
magnetic field occurrence does not depend on the
→ spectral type. Because
these magnetic fields seem to be stable over long time-scales and their
strength does not seem to correlate with known stellar properties, it
is assumed that they are of fossil origin
(→ fossil magnetic field)
and are frozen into the → radiative envelope
of the stars.
The fields are those of the birth
→ molecular clouds, partly trapped inside
the → pre-main sequence star
during the cloud → collapse
phase, possibly further enhanced by a
→ dynamo effect in the early fully convective
stellar phase.
Typically, the polar field strength ranges from about a
hundred → Gauss up to several kiloGauss.
However, some weaker fields,
below 100 G, have recently been detected.
The stellar magnetic field influences many different regions of the
star with various effects. In the deep interior of the star, the field
influences the internal → mixing
of the star and this affects the size
of the → convective overshooting
region, changing the lifetime of
the star by decreasing the amount of fuel for nuclear
burning. Magnetic stars can also confine their
→ stellar winds, due to
their strong magnetic fields, into a
→ magnetosphere, which slows down
the → rotational velocity of the star. This
→ magnetic braking is
an efficient mechanism for
→ angular momentum transport.
At the stellar surface, the magnetic
fields can create and sustain areas of chemical over- or
under-abundances and/or large temperature differences, which are
called spots
(Buysschaert et al., 2016, astro-ph/1709.02619).

A star visible without a telescope. In
principle, stars down to about sixth magnitude are visible to the
naked eye under ideal conditions, but this
depends on the individual, the location, and the conditions of the
observation.

An extremely compact ball of matter created from the central core of
a star that has collapsed under gravity to such an extent that it consists
almost entirely of → neutrons. Neutron stars result from two
possible evolutionary scenarios: 1) The → collapse
of a → massive star during a
→ supernova explosion; and 2) The accumulation of mass by a
→ white dwarf in a → binary system.
The mass of a neutron star is the same as or larger than the
→ Chandrasekhar limit (1.4
→ solar masses). Neutron stars are only about 10 km
across and have a density of 1014 g cm-3, representing
the densest objects having a visible surface. The structure of neutron stars
consists of a thin outer crust of about 1 km thickness composed of
→ degenerate electrons and nuclei, which becomes progressively
neutron rich with increasing depth and pressure due to
→ inverse beta decays. In the main body
the matter consists of → superfluid neutrons in equilibrium
with their decay products, a few percent protons and electrons.
Neutron stars have extremely strong magnetic fields, from 3 x 1010
to 1015 gauss. As of 2010 more than
2000 neutron stars have been catalogued, which show a large variety of
manifestations, mainly → pulsars.

A star that is never seen above the horizon from a given position. These stars are
located between the celestial pole and a diurnal circle with an angular distance
larger than the altitude of the pole.

A star that is always seen above the horizon from a given position. These stars are located
between the celestial pole and a diurnal circle with an angular distance smaller than the
altitude of the pole. Same as → circumpolar star.

A luminous, hot, blue star whose spectrum is dominated by the lines of hydrogen,
atomic helium, and ionized helium; also known as O-type star.
This is the earliest → spectral type
and the only → main sequence star in which
ionized helium is present. The → effective temperatures
of these stars range from about 30,000 K to 50,000 K, their luminosities from
50,000 to 1,000,000 times that of → solar luminosity, and their
masses from about 20 to 100 → solar masses.
The hottest O-type stars display high ionization emission features such as N III and He II,
→ Of star. They are divided into subtypes O2, the hottest, to O9.7,
the coldest. O-type stars are relatively rare, for each star of 100 solar masses there are
106 stars of solar mass. They are relatively short-lived since
they spend only a few million years on the main sequence. The brightest O-type star
in the sky visible with naked eye is → Alnitak.
For prominent Galactic O stars see → HD 93129.

O, letter of alphabet used in the Harvard spectral classification;
→ star.